Field of the Invention
The present invention relates to a pressure control valve applied in the fluid pressure circuit used in an industrial vehicle and a control valve system including a pressure control valve as a constituent element.
Description of the Related Art
Conventionally, referring now to FIG. 5 and as set forth below, a balance piston type valve is known as a pressure control valve and is used in a fluid pressure circuit used for industrial vehicles. This type of pressure control valve V is installed between an input port a1c receiving an operation fluid and a tank port a1d continuously connected to a tank for storing operation fluid and valve V comprises a main valve element a6 and a valve seat a7a in which the main valve element a6 can seat and also a main valve unit a4 formed by utilizing a valve storage body a7 having a pilot chamber a7b in which the main valve element a6 is slidably stored, the pilot valve element a8, and a pilot valve unit a5 formed by utilizing a pilot valve seat member a9 having a pilot valve seat a9a in which the pilot valve element a8 can seat and fixedly installed in the main valve unit a4. Both the main valve element a6 and the pilot valve element a8 are biased by a biasing means a101, a102 (such as a spring) to seat respectively in the main valve seat a7a and the pilot valve seat a9a. Then, when both the main valve unit a4 and the pilot valve unit a5 are in the closed valve state, fluid pressure inside the input port a1c is operative to the pilot valve element a8 through the inside of the pilot chamber a7a. 
Then, when the inside pressure of the input port a1c becomes higher than the predetermined pressure, fluid pressure operative to the pilot valve element a8 is stronger than the biasing force by the biasing means a102 and the pilot valve element a8 separates from the pilot valve seat a9a and the pilot valve unit a5 will be in the open valve state. Then, as the inside of pilot chamber a7b continuously connects to the tank port aid, the differential pressure between the inside of input port a1c and the inside of pilot chamber a7b is generated and the force due to the differential pressure is stronger than the biasing force by the biasing means a101 so that the main valve element a6 leaves from the main valve seat a7a and the main valve unit a4 will also be in the open valve state.
Accordingly, while the pilot valve unit a5 is in the open valve state and the main valve unit a4 is in the closed valve state, operation fluid flows through the gap between the pilot valve seat a9a and the pilot valve element a8 of the pilot valve seat member a9, but unfortunately the central axis of the pilot valve seat a9a and the central axis of the pilot valve element a8 may not be always coaxial and also the back-and-forth direction of the pilot valve element a8 may not be the same as the extending direction of the central axis of the pilot valve element a5. Therefore, in the case of these situations the pilot valve element a8 is eccentric relative to the pilot valve seat a9a and operation fluid flows through the gap, the problem takes place, wherein as a result the pilot valve element a8 can vibrate due to the fluid pressure of fluid and collides with the pilot valve seat member a9 to make abnormal noise, causing premature wear, leakage and other defects.
One construction in an effort to solve such problems is disclosed in which a guide to keep slidably the pilot valve element is installed to make the back-and-forth direction of the pilot valve element as the same as the extending direction of the central axis of the pilot valve element (e.g., Patent Document 1, the entire contents of which are incorporated by reference).
However, unfortunately according to the constitution disclosed in Patent Document 1, since the guide member must be additionally installed and modified, the number of parts and assembly processes increase so that an increase of the production cost can be raised as another problem.